It's not an anomaly, it's normal. It doesn't surprise me that an autofocus camera might have trouble determining the exact focus point when using a 15mm lens. I don't have a lot of luck doing that by eye with a 15mm lens either, especially at an aperture smaller than wide open. If the camera achieves correct focus with a 15mm lens only at wide open, f/3.5 in this case, it's doing the best it can. That's why lenses are made to be able to focus wide open, to begin with. If it is important to have the plane of exact focus somewhere specific, you should focus and shoot wide open with a wide (or in this case, ultra-wide) angle lens.
Try this. Focus wide open. Then press the DOF preview while still looking through the viewfinder. If you are at some smaller aperture than wide open, you should see a difference. The cameras auto-focus system also sees a difference when the lens it stopped down. Len --- -----Original Message----- From: Graham Lyth [mailto:[EMAIL PROTECTED]] Sent: Tuesday, March 05, 2002 5:49 AM To: [EMAIL PROTECTED] Subject: Interesting Z-1 AF behaviour with wideangle lens (longish) Hi all, Been lurking here for a while, nice to post for a change. This morning, I've been out in the back garden taking some test shots with a new Sigma 15-30 f/3.5-4.5 EX fitted to my Z-1, and I've come across some interesting AF behaviour that I've not noticed before. Testing at 15mm, I noticed that the AF wasn't focusing precisely on the subject - it was focusing a few feet in front. I played for a few minutes, with the camera on a tripod pointing towards the house about 12 feet away, and concluded that the position of focus varied with the aperture set. Wide open, the focus was accurate. As I stopped down, the point of focus moved towards the camera. Presumably, the camera was making a judgement on where to focus in order to maximise depth of field whilst keeping the subject sharp. I got the same result regardless of whether I changed the aperture via the ring or the body. The Z-1 is set to use the standard program line, not the DOF variant. I've not noticed this before and haven't seen it discussed - can anybody shed any light? Graham -- - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
